Apparatus for providing higher order mode compensation in horn antennas

ABSTRACT

The unwanted higher order mode power that occurs in flared antenna horns is compensated for by inserting flare sections into the antenna horn feed structure. The feed structure flares also generate higher order mode power and are positioned and constructed in such a manner that the higher order mode power so generated is substantially equal to and in phase opposition with the higher order mode power occurring in the antenna horn. Various embodiments utilize single outward, double outward and double inward flares.

United States Patent 1191 Blume APPARATUS FOR PROVIDING HIGHER ORDER MODE COMPENSATION IN HORN ANTENNAS [75] Inventor: Alan E. Blume, Trotwood, Ohio [73] Assignee: The United States of America as represented by the Secretary of the Air Force, Washington, DC.

221 Filed: Feb. 22, 1974 21 Appl. No.: 445,048

Related US. Application Data [62] Division of Ser, No. 360,517, May 15, 1973,

abandoned.

[52] US. Cl. 343/786; 333/21 R [51] Int. Cl. H01q 13/00 [58] Field of Search 343/786; 333/21 R [56] References Cited UNITED STATES PATENTS 3,662,393 5/1972 Cohn 343/786 1451 July 22,1975

Primary Examiner-Eli Lieberman Attorney, Agent, or Firm-Joseph E. Rusz; Willard R. Matthews, Jr.

[5 7] ABSTRACT The unwanted higher order mode power that occurs in flared antenna horns is compensated for by inserting flare sections into the antenna horn feed structure. The feed structure flares also generate higher order mode power and are positioned and constructed in such a manner that the higher order mode power so generated is substantially equal to and in phase opposition with the higher order mode power occurring in the antenna horn. Various embodiments utilize single outward, double outward and double inward flares.

2 Claims, 3 Drawing Figures 1 APPARATUS FOR PROVIDING HIGHER ORDER MODE COMPENSATION IN HORN ANTENNAS This is a division of application Ser. No. 360,517, filed May 1973, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to microwave antenna horns, and more particularly to a method and means for eliminating the unwanted higher order mode power that results from the flared geometry of such antenna horns when fed from oversized waveguide.

The use of a flared rectangular antenna horm with oversized rectangular waveguide results in the generation of unwanted higher order mode power in the horm throat. For example, LSE mode power is formed in the horn throat due to a horns E-plane flare geometry. This higher order mode power may radiate so as to make the first or other sidelobe of the overall antenna pattern higher relative to the beam peak than the pattern carried by the dominant TE, mode. Accordingly, there currently exists the need for a method and means for reducing the magnitude of these sidelobes. The problem is further complicated in certain applications in which horn redesign, excessive feed circuit length, or reduced power handling capacity are not tolerated.

The present invention is directed toward satisfying such a need. The invention also provides various alternative embodiments that permit the selection of an appropriate higher order mode compensation device for any particular application.

SUMMARY OF THE INVENTION The basic concept of the invention comprehends forming the correct amount of the higher order mode power to be compensated for by means of flares located in the antenna feed circuit and bringing it to the point in the horn throat where the unwanted higher order mode power is formed in the correct phase so that cancellation occurs.

A first embodiment of the invention utilizes double outward E-plane flares. It can be used with an E-plane flared rectangular horn and oversized rectangular waveguide and requires a one-and-one-half wavelength extension of the feed circuit.

A second embodiment of the invention utilizes double inward E-plane flares. It can also be used with an E-plane flared rectangular horn and oversized rectangular waveguide. It is, however, limited in peak power handling capacity.

A third embodiment of the invention utilizes a single outward E-plane flare and when used with oversized rectangular waveguide requires redesign of the antenna horn. It has no peak power handling limitation and increases the feed circuit length by only one-half wavelength.

The same principles can be used to compensate H- plane flared rectangular antenna horns, or horns flared in both planes (i.e., pyramidal). They may also be used to compensate horns which are rectangular in crosssection at the throat and change to circular between the throat and the mouth.

It is a principal object of the invention to provide new and improved methods and means for eliminating unwanted higher order mode power in E-plane and H- plane flared rectangular antenna horns.

It is another object of the invention to provide a higher order mode compensated antenna that utlizes oversized waveguide and a conventional E-plane or H- plane flared antenna horn.

It is another object of the invention to provide a higher order mode compensated antenna that utilizes oversized waveguide and a conventional E-plane or H- plane flared antenna horn.

It is another object of the invention to provide a higher order mode compensated antenna that does not require excessive feed structure length.

It is another object of the invention to provide a higher order mode compensation for an E-plane or H- plane flared horn antenna without limiting its peak power handling capability.

These, together with other objects, advantages and features of the invention will become more apparent from the following detailed description when taken in conjunction with the illustrative embodiments in the accompanying drawings.

DESCRIPTION OF DRAWINGS FIG. 1 is a sectional view of an embodiment of the invention employing double outward E-plane flares;

FIG. 2 is a section view of an embodiment of the invention employing double inward-E-plane flares; and

FIG. 3 is a section view of an embodiment of the invention employing a single outward E-plane flare.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is illustrated thereby the double outward E-plane flared embodiment of the invention. It comprises a flared rectangular horn 4, waveguide transmission line segment 5, E-plane flared rectangular waveguide segments 6 and 7, and waveguide transmission line 8.

In operation, the outward flare at A of angle 0 causes a certain amount of the transmitted TE mode power to be converted into LSE mode power. (The LSE mode is a combination of the TE and TM modes.) The inward flare at B, of angle 20, creates a larger amount of LSE mode power (more than twice as great) of opposite sign. The outward flare at C of angle 0 generates approximately the same amount of LSE as at A and of the same sign. In addition there is the LSE mode created in the horn throat of angle 6', which has the same sign as that at A and C. If the phase length difference between the TE mode and the LSE mode is from A to B, from B to C, and from C to D, then all the LSE created by the double outward flare will be in phase and will be out of phase with the LSE in the horn throat.

The equation for the field strength of the LSE mode at A and C is b, is the E-plane height at A and C; b is the E-plane height at B; L, is the distance between A and B, and B and C.

The equation for the field strength of LSE at B is The phase difference between TE and LSE modes is L is the length of the flare; a is the H-plane dimension or width of the waveguide; b is the E-plane dimension or height.

A secondembodiment of the invention that utilizes a double inward E-plane flare configuration is illustrated by FIG. 2 of the drawings. It consists of a flared rectangular born 4, waveguide transmission line 8, inwardly flared rectangular waveguide segment and outwardly flared rectangular waveguide segment 9 connected as shown. By way of example, for a system using oiversized 8 X 5.36 inches waveguide L is made as long as possible, certainly over 24 inches. If b is about 4.5 inches, then the LSE mode generated at A will be in the order of 2 percent in voltage of the incident TE mode for A in the order of 4 inches and is almost negligibl'e. Since h is 5.36 inches (the E-plane'dimension of the horn throat), the LSE mode generated at B will be in the order of 10 percent in voltage for L in the order of 5 inches and b roughly 1 inch less than 12 6 will then be about 6. The effective flare angle at C is then 05 6 or about 7?, and the LSE mode generated at C is only about 0.13. If L is half a beat wavelength long, then the net LSE mode propagating into the horn is quite small, since the LSE mode generated at B will largely cancel that generated at C. Furthermore, since L is only a half wavelength at center frequency, its frequency sensitivity may be relatively small.

A third embodiment of the invention is illustrated by FIG. 3 of the drawings. This embodiment comprises E- plane flared antenna horn 12 in combination with the single outwardly flared waveguide segment 11. The nonflared end of segment 11 is adapted to mate with oversized waveguide transmission line 8.

The flare segments and horn must be designed together, because the optimum value of L will probably notbe a half wavelength. L should be long enough so that the "net LSE mode traversing the antenna horn should be in phase with the TE mode in the horn mouth at or near center frequency. Then the 180 relative phase reversal in the first sidelobe region will cause partial cancellation and could result in a lower sidelobe level.

The calculations are rather involved, because the horn length affects the optimum value of L and 0 and the values of L and 6 affect the horn length. Thus a simultaneous analytical solution may be used, or alternatively, optimum parameters may be determined empirically. Here, it appears that overcompensation should be used; the LSE mode generated at A should be greater than the LSE mode generated at B. L will be roughly a half wavelength; since the LSE mode is well above cutoff from A to B, the frequency sensitivity of the phase length of L should be roughly one-third of the three lengths used for the double outward E-plane flare. 1f the LSE mode generated at A is to be considerably more than that generated at B, 0, should be approximately percent of 0 Since 0 will be approximately 13, 0, will be approximately 10 and b will be in the order of 10 inches if 5.36 inch high waveguide is used.

While the invention has been described with reference to various presently preferred embodiments, it is understood that the words which have been used are words of description rather than words of limitation and that changes within the purview of the appended claims may be made without departing from the scope and spirit of the invention in its broader aspects. In particular, although compensation methods for the LSE mode generated at the throat of E-plane flared rectangular horns have been shown, the same principles may be used to compensate for the TE mode generated at the throat of H-plane flared rectangular horns and for the LSE and TE modes generated at the throat of pyramidal rectangular horns.

What is claimed is:

1. An LSE mode compensated antenna comprising a first rectangular waveguide segment having an inward E-plane flare, the input end thereof being adapted for connection to a rectangular waveguide transmission line,

a second rectangular waveguide segment having an outward E-plane flare having its nonflared end connected to the output end of a said first waveguide segment, and

a rectangular antenna horn having an outward E- plane flare connected at its input end to the flared end of said second waveguide segment,

said second waveguide segment having length and flare angle values that effect the generation and phase reversal of LSE mode power.

2. An LSE mode compensated antenna as defined in claim 1 wherein said second waveguide segment has a length of substantially one-half wavelength. 

1. An LSE12 mode compensated antenna comprising a first rectangular waveguide segment having an inward E-plane flare, the input end thereof being adapted for connection to a rectangular waveguide transmission line, a second rectangular waveguide segment having an outward E-plane flare having its nonflared end connected to the output end of a said first waveguide segment, and a rectangular antenna horn having an outward E-plane flare connected at its input end to the flared end of said second waveguide segment, said second waveguide segment having length and flare angle values that effect the generation and phase reversal of LSE12 mode power.
 2. An LSE12 mode compensated antenna as defined in claim 1 wherein said second waveguide segment has a length of substantially one-half wavelength. 